Sizing Key Mortises (was inconsistent rail pins)

[Kenneth Jankura]

List,

We've discussed this periodically for years. I posted about an  
experiment I did for our chapter with a 1" drill bit and lots of 3/4"  
x 6" x 6" blocks. It helps to think large and simple so as to get the  
concepts.
Drill a hole. Soak the board in water. Drill bit fits through hole  
without touching the sides.
Drill a hole. Oven dry board. Drill bit is so big you'll never  
believe it once went through that tiny hole.
I did many configurations, like drill hole while wet, dry just a  
little, etc.  Every instance follows the rule that the hole gets  
smaller when humidity is less. (Actually because of grain  
orientation, the hole becomes oblong and smaller.)
This presumes wood moisture content equilibrium, that is, in times of  
rapid humidity increase, you might have a hole become slightly  
smaller until the wood achieves equilibrium. Picture this. Drill a  
hole, drop the board in water, the cells right around the hole swell  
immediately and the drill bit won't fit through. When the whole board  
takes on water equally, the drill bit will fit through the hole  
without touching the sides. Try it.  Again, this is a gross over- 
simplification, but in general, holds true like a law of nature.
Tuning pin tightness is a COMPLETELY DIFFERENT scenario and doesn't  
follow exactly what normal wood would do. Never confuse pinblock  
behavior and normal wood behavior!
The cross ply grain orientation in the pinblock keeps the wood stable  
except right along the outside edges where the wood cells are free to  
shrink or enlarge (this includes the 'outside edges' around each  
pin). So in low humidity the wood cells shrink only along the 'edges'  
and the pins get loose. Vice versa in the summer. If the pinblock  
were made out of one piece of wood, with no cross ply, the pins would  
be tight in the winter and loose in the summer, just as what happens  
with all the other wood.
And the wood technician who doesn't get this should read Hoadley  
again for a refresher. It has been my experience that Steinway teflon  
bushing flanges click like crazy in the summer as they flop around in  
the humidity-enlarged flange holes, and they get really tight in the  
winter as the holes shrink and squeeze the bushings.
I hope this helps clear things up a little.

Ken Jankura RPT
Newville PA

On Sep 12, 2006, at 6:07 AM, Ric Brekne wrote:

> Hi folks
>
> I always liked the balloon analogy.  Take a balloon and and blow it  
> half way up. Then with a tush pen draw a bunch of equal sized  
> circles all over its surface.  When you then further expand the  
> balloon what happens to the circles (holes) ?  And what happens  
> when you let the air out a bit (contraction).
> This analogy came up a few years back when we were scatching our  
> heads about comparing summer/winter tuning pin tightnesses.
>
> Cheers
> RicB